The enzyme catalyzes the asymmetric condensation of two molecules of delta-aminolevulinic acid to form porphobilinogen, which is the first common reaction in the biosynthesis of the tetrapyrroles. The enzyme shows a variable requirment for an essential zinc ion in different species which may indicate phylogenetic diversity in its chemical reaction mechanism. Among those enzymes that do not use the catalytic zinc, some appear to use a monovalent cation, some may use magnesium and others may not use any metal ions at all. On the other hand there is an allosteric Mg ion that is present in about 90% of the enzymes PMID15381398.